I'm a geochemist. In the past ten years I've fixed mass spectrometers, blasted sapphires with a laser beam, explored for uranium in a nature reserve, and measured growth patterns in fish ears, and helped design the next generation of the world's most advanced ion probe. My main interest is in-situ mass spectrometry, but I have a soft spot in my heart for thermodynamics, drillers, and cosmochemistry.

Wednesday, October 23, 2013

Sciency Thoughts has a post up on recent high-precision 39Ar/40Ar
dating of the Toba supervolcano in Indonesia. Unfortunately he seems a bit confused about
the technique.

Argon has three naturally occurring isotopes: 36Ar,
38Ar, and 40Ar. Potassium also has three isotopes, 39K,
40K, and 41K. One of these isotopes, 40K, is radioactive, with a half life of about 1248 million years, and one of its
stable decay products is 40Ar.

In the universe, and
in Jupiter and the Sun locally, 36Ar is the most abundant argon
isotope, followed by 38Ar. In
the cosmic scheme of things, 40Ar is so rare that we don’t even know
what its overall abundance is.

However, Earth is a rocky planet. It was not able to hold onto much gas during
its formation, so there is very little 36Ar and 38Ar
here. Earth has lots of potassium
though, so almost all the Ar in the atmosphere is 40Ar, which is the
decay product of 40K.

In a potassium-bearing mineral, the 40K decays
into 40Ar, so you can measure the ratio of these two isotopes to
figure out how old the mineral is.

The problem is that it is technically very difficult to
measure a potassium argon ration accurately, because one is a reactive solid,
and the other is an inert gas. They
require different sorts of ion sources, different mass spectrometers, and there
are all sorts of chemical effects that complicate the measurement.

If you want an accurate ratio, it is much easier to measure
isotopes of the same element.

So for 39Ar-40Ar dating, what happens
is that the mineral of interest is put into a nuclear reactor and bombarded by
neutrons. Some of the 39K
(the most abundant stable potassium isotope) absorbs a neutron, ejects a
proton, and transmutes into radioactive 39Ar. 39Ar has a half-life of a few
hundred years, and is virtually non-existent in nature. So as
long as you know your nuclear 39K to 39Ar conversion
ration well, this method allows you to use the 39Ar as a proxy for 39K. The handy thing is that because it is argon,
not potassium, it behaves chemically just like the other naturally occurring
argon isotopes, so you can measure it in a gas source mass spectrometer much
more accurately than you can measure the chemically different 39K
and 40Ar.

The initial 39K-40K ratio doesn’t very
much in nature, and is taken as constant (I think- I’ve never actually done
Ar-Ar). But the take-home point is that 39Ar-40Ar dating
is not its own decay system. It is the 40K-40Ar
decay system, but using a nuclear reactor to change some of the potassium into
an unstable argon isotope to make the nuts and bolts of measuring it easier.

2 comments:

You might be interested to know that the Storey et al. date is disputed - an even more recent Ar-Ar date from the SUERC lab has been published by Darren Mark and his colleagues in Quaternary Geochronology (http://dx.doi.org/10.1016/j.quageo.2012.12.004).

The main difference is in how the Ar-Ar system is "calibrated" (that is - decay rate + flux monitor age). It might also interest Dr. Lemming that such a discrepancy also exists for the Bishop Tuff.

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